Radar signals serve as a useful sensing technique for observing objects, such as persons, animals and/or vehicles. If the object or a part of the object has rotation or translation in addition to the body motion, it might induce a frequency modulation and amplitude modulation on the radar return signal. These modulations are the so-called micro-Doppler or micro-motion effects and is observed in the 2-D time-speed space. By analyzing radar signals such objects can also be observed in situations where they are not or less visible for the human eye, e.g. behind walls or at great distances.
The challenge of motion estimation with radar, e.g. human motion estimation, has existed since the beginning of radar and has continued in modern radar applications. However, the technology has been hindered by performance and cost barriers typically associated with the development of modern radar applications. In public applications radar becomes more and more interesting and available at low cost. In many applications the radar shall provide location, speed and reflected power of stationary and moving objects. By combining a number of low cost radar sensors a non-coherent radar sensor network can be built which provides additional object information. This non-coherent radar sensor network uses radar sensors without synchronization of frequency or phase to keep the system at low cost.
Several low cost commercial off-the-shelf (COTS) radar sensors have been developed in recent years for public applications. One category is the automotive radar. These radars operate under all weather conditions, day and night. An object position may be determined by trilateration of the measured distances of individual range profiles. An extremely high range accuracy of each radar sensor inside the network is needed for sufficient trilateration angle accuracy. The classification algorithm classifies the target in six groups: cars, cyclists, pedestrians, trees, traffic signs and a group of persons. Classification is based on range, cross-range, speed and power.
Another category of low cost radar sensors is developed for application areas like traffic monitoring, automatic door openers, alarm equipment, sanitary equipment, sport applications etc. Standard products with radar frequency (RF) front-end and antenna are available with relatively low prices. Here radar systems are no longer price determinative and applications with multiple radars can be built. The radar systems in these categories satisfy government regulations and can be applied in public areas.
Low cost radar sensors have however also disadvantages. They have not been developed for a specific application and as a result there is generally no parameter match and the system is fixed. Parameters such as the antenna beam width, transmitter frequency, sweep repetition frequency, transmitter power, angle measurements etc. gives the possibility to extract specific target information. The technical performance is related with the price such as the noise level, the Frequency Modulated Continuous Wave (FMCW) radar sweep linearity and parameter settings. A number of these shortcomings can be solved in the post-processing but others are more fundamental and can not be solved that way. Angle measurements used in radar imaging can be realized in several ways, antenna beam scanning radar sensors, radar sensors that depend on the phase information for angular resolution and by combining information from multiple radar sensors. Low cost radars can be used in this last group.
Characteristics of these low cost radar sensors are the unsynchronized transmitted frequency and phase and transmitted power. In case of a FMCW radar sensor network the range-speed responses are time synchronized. Trilateration can be used by combining information from multiple noncoherent radar sensors. Non-coherent means the phase information can not be used for angular information. To locate an object in three dimensions a minimum of three measurements with different radar sensor locations are necessary. This approach works if the object behaves as a point target when there is only one object and the object is illuminated by each of the radars. The approach is very sensitive for distance errors when the baseline is small. Humans give an extended response and do not satisfy to these assumptions at all. A human contains independently moving reflection points and the illuminated part depends on the radar senor network. Besides these a human causes multi-path reflections and scintillation. Multi-path is the phenomenon where the phase interferes between the direct echo and the indirect echo reflected by the environment or another human body parts. Scintillation results from two or more direct echoes interfering with each other. Both multi-path and scintillation make it impossible or very difficult to distinguish individual human body parts.